Consider a Code Division Multiple Access (CDMA) system that uses orthogonal Walsh functions to separate users on the downlink along with a base station specific covering code. It is well known that “downlink” refers to the communication path or paths from a base station to a mobile terminal or station (hereinafter referred to as a “mobile”), as compared to the “uplink” which is the communication path or paths from a mobile to a base station. For a system with K mobiles receiving signals from a common base station, the transmitted signal on a single antenna may be represented as:
                              x          ⁡                      (            t            )                          =                              (                                                            ∑                                      i                    =                    1                                    K                                ⁢                                                                            P                      i                                                        ⁢                                                            s                      i                                        ⁡                                          (                      t                      )                                                        ⁢                                                            w                      i                                        ⁡                                          (                      t                      )                                                                                  +                                                                    P                    p                                                  ⁢                                                      w                    0                                    ⁡                                      (                    t                    )                                                                        )                    ⁢                      p            ⁡                          (              t              )                                                          (        1        )            where Pi is the power transmitted to the ith mobile, si(t) and wi(t) are the data signal and unique Walsh function intended for the ith mobile, respectively, Pp is the power of the pilot signal which uses Walsh function 0, and p(t) is the covering code for the base station of interest. Further, the Walsh functions are orthogonal and repeat every symbol time Ts, i.e.:
                                          ∫            0                          T              s                                ⁢                                                    w                i                            ⁡                              (                t                )                                      ⁢                                          w                j                            ⁡                              (                t                )                                      ⁢                          ⅆ              t                                      =                  {                                                    1                                                              i                  =                  j                                                                                    0                                                              i                  ≠                  j                                                                                        (        2        )            At the mobile, the following signal is received on a single antenna:y(t)=h(t)x(t)+n(t)  (3)where h(t) is the complex multiplicative distortion caused by the wireless channel and n(t) is thermal noise. Mobile i correlates the received signal with the ith Walsh function during the kth symbol interval after uncovering to achieve the decision statistic zI[k]:
                                                                                                              z                    i                                    ⁡                                      [                    k                    ]                                                  =                                                      ∫                                                                  (                                                  k                          -                          1                                                )                                            ⁢                                              T                        s                                                                                    k                      ⁢                                                                                          ⁢                                              T                        s                                                                              ⁢                                                            y                      ⁡                                              (                        t                        )                                                              ⁢                                                                  p                        *                                            ⁡                                              (                        t                        )                                                              ⁢                                                                  w                        i                                            ⁡                                              (                        t                        )                                                              ⁢                                          ⅆ                      t                                                                                                                                              =                                                                                                    P                        i                                                              ⁢                                          h                      ⁡                                              [                        k                        ]                                                              ⁢                                                                  s                        i                                            ⁡                                              [                        k                        ]                                                                              +                                      n                    ⁡                                          [                      k                      ]                                                                                                                                       (        4        )            where h[k] represents the cumulative effect of the channel h(t) over the kth symbol interval, and si[k] is the kth transmitted symbol for the ith mobile.
The transmitted symbol can be recovered by using an estimate of the channel, ĥ[k], obtainable from the pilot channel, i.e.:ŝi[k]=f(zi[k]ĥ*[k])  (5)where f(•) is an appropriate decision function. Assuming a flat, Rayleigh faded channel, in the absence of fast, accurate power control, the resulting performance of the link will be rather poor due to the lack of diversity. As a result, it is desirable to have a second antenna at the receiver to allow diversity reception, improving performance considerably. However, mobile handsets do not easily allow a second antenna to be added. Thus, methods of achieving diversity performance from the transmitter have been proposed.
One method of achieving diversity performance is to transmit the same signals on multiple carriers. However, this is wasteful of the one resource that cannot afford to be wasted in mobile communications, namely, bandwidth. A second more reasonable alternative is delay diversity. This method purposely causes multipath by transmitting the signal twice from the base station with the second transmission delayed in time by several chips and occurring on a separate antenna. By dividing power over two transmissions transmit power is not increased, but time diversity is provided which can easily be exploited by the mobile's Rake receiver with no required changes. However, the scheme is ultimately limited by the self-interference caused by this intentional multipath. Since all user signals are transmitted synchronously, this multipath interference can be quite large, especially for a moderate to heavily loaded system.
Other techniques have been proposed for attempting to improve diversity in the area of space-time coding. For example, a simple two-branch transmission diversity technique is described in Siavash M. Alamouti, “A Simple Transmit Diversity Technique for Wireless Communications,” IEEE Journal On Select Areas In Communications, Vol. 16, No. 8, Oct. 1998, the disclosure of which is incorporated herein by reference. Further, the use of channel codes for improving the data rate and the reliability of communications over fading channels using multiple transmit antennas is described in Vahid Tarokh et al., “Space-Time Codes for High Data Rate Wireless Communication: Performance Criterion and Code Construction,” IEEE Transactions On Information Theory, Vol. 44, No. 2, Mar. 1998, the disclosure of which is incorporated herein by reference. Still further, a technique for coding messages for transmission on the downlink so as to use multiple transmitting antennas for improved reception in fading environments is described in U.S. provisional application Serial No. 60/114,621, filed on Jan. 4, 1999, entitled: “Space-Time Spreading Method of CDMA Wireless Communication,” which is the basis for a U.S. non-provisional application filed on Apr. 2, 1999 having the same title.